(1) Field of the Invention
The present invention relates to a nitride electronic device intended for power switching as represented by an inverter used for an electric vehicle and a home electric appliance, and also relates to a nitride optical device as represented by a blue/white light-emitting diode (LED) and a laser device.
(2) Description of the Related Art
Recently, a nitride semiconductor has been under intense research and development as a material for high-output power devices or blue light-emitting devices, to the extent that an optical device using a nitride semiconductor, in particular, has already been introduced into the market.
A Modulation Doped Field-effect Transistor (MODFET) applying an aluminum gallium nitride (AlGaN)/gallium nitride (GaN) heterojunction is studied as an electronic device. The most significant difference between such MODFET and a gallium arsenide (GaAs) MODFET is that the former MODFET can realize a sheet carrier density ten times as much as that of the GaAs MODFET without doping, with impurities, an AlGaN layer which is a Schottky layer. The mechanism of carrier generation is that two-dimensional electrons are accumulated in a boundary between AlGaN and GaN, which is caused by a polarization generated due to a piezo effect in the AlGaN layer because of the stress between AlGaN and GaN due to the lattice mismatch, as well as a voluntary polarization in AlGaN. Therefore, stress is a very important parameter for the MODFET applying the AlGaN/GaN heterojunction, and thus, a relationship between sheet carrier density and the stress between AlGaN and GaN is vigorously researched. For example, sheet carrier density of two-dimensional electron gas is quantitatively calculated based on stress, as described in pp. 3222-3233, Vol. 85, Journal of Applied Physics (1999) written by O. Ambacher et al.
As is represented by the example shown in the above-mentioned reference, the conventional AlGaN/GaN heterojunction is formed by stacking layers in a direction of C-axis. This is because, with such formation of heterojunction, namely, with the formation of an AlGaN/GaN heterojunction on C-plane, the effect of polarization unique to nitride-based compound semiconductor can be obtained.
In contrast, Japanese Laid-Open Application No. 2002-76329 discloses a structure in which AlGaN/GaN heterojunction is formed on A-plane of a sapphire substrate and a gate direction is paralleled to a direction of C-axis of the sapphire substrate. Thus, it is possible to make effective use of low conductivity in the C-axis direction of the sapphire substrate, and thereby to achieve high-speed operation in an electronic device. Such case also allows the AlGaN/GaN heterojunction, being a device region, to be formed by stacking layers in the C-axis direction due to an epitaxial relationship between the sapphire substrate and a GaN layer, although the main surface of the sapphire substrate on which the AlGaN/GaN heterojunction is to be formed is the A-plane. That is to say that the use of the AlGaN/GaN heterojunction on the C-plane is the same as described in the cited reference, Journal of Applied Physics. In addition, the Japanese Laid-Open Application No. 2002-76329 does not explicitly mention a relationship between a gate direction of an electronic device and a crystal direction of nitride semiconductor material.
Japanese Laid-Open Application No. 2001-160656 discloses a field-effect transistor (FET) in which A-plane or M-plane of a material containing crystal which has a wurtzite structure is used as a main surface of a substrate, and a length direction of a gate electrode is placed vertical to C-axis so that a direction of current is made parallel to the C-axis. With such FET, a dispersion of carriers due to dislocation is reduced so that the FET with excellent electric characteristics can be realized.